1. Field of the Invention
This invention relates to a computer readable medium having a program for permitting a computer to perform image processing, and image processing method.
2. Description of the Related Art
In recent years, image processing and visualization technologies requiring complicated calculation have been used on a day-to-day basis with the progression of the image processing technology using a computer. Particularly, in the medical field, medical diagnosis is widely conducted using a CT (computed tomography) apparatus, an MRI (magnetic resonance imaging) apparatus, or PET (positron emission tomography) for enabling the user to find a focus at an early stage by visualizing the inside of a living body.
A method called volume rendering is known as a method of providing a three-dimensional image of the inside of an object. In volume rendering, a virtual ray is applied to a set of three-dimensional voxels (micro volume elements), thereby projecting an image onto a projection plane. A large number of volume rendering calculation methods are devised to meet various usages; representative volume rendering calculation methods include ray casting method, MIP (maximum intensity projection) method, and MPR (multi planer reconstruction) method.
FIGS. 13A and 13B are explanatory diagrams of a method of related art for improving the image quality of a display image stepwise. FIG. 13A shows an image displayed first when an image is displayed. The first displayed image is of low image quality because high-speed processing is required for displaying the image. FIG. 13B shows a display screen after a lapse of some time from the state in FIG. 13A; it takes time until the image is displayed although the image is of high image quality. To process and display an image, generally it takes time to generate a high-quality image because the calculation amount is large (large calculation amount image), and it does not take time to generate a low-quality image because the calculation amount is small (small calculation amount image), although it depends on the image quality optimization degree and the subjective element.
Thus, to observe the diseased area by the image of the inside of a human body photographed by a CT apparatus and the like, when the user (doctor, etc.,) searches for the observation part of the diseased area or is operating the apparatus to change the scale factor, a coarse image as shown in FIG. 13A is displayed with a good response, and when the user stops operating the apparatus and observes the observation part in detail, a high-quality image as shown in FIG. 13B is generated and is displayed.
On the other hand, as an image generating technology to improve the ease of use in displaying an image, for example, progressive JPEG is available as one of the expanded specifications of JPEG, which is an image file format often used in the Internet, etc. For a usual JPEG image, an image of a predetermined resolution is displayed gradually from the top of a screen as the image is downloaded into a computer of a user; when an image generated in the progressive JPEG format is downloaded, a vague image appears on the whole screen at first, and the image on the whole screen becomes clear gradually with the progression of downloading the image. Thus, the user can grasp an approximate image even while the image is being downloaded. (Refer to “JPEG—from concept to implementation by C++ (JPEG—gainen'kara C++ niyoru jissoumade)” written by Keiwa-kui, issued by Softbank Shuppan Jigyoubu, Sep. 1, 1998, p. 222-224.)
By the way, in medical diagnosis using an image, generally the user conducts a diagnosis using a large calculation amount image of high image quality, and thus the large calculation amount image of the observation target needs to be generated again and again for display to provide information required for diagnosis. Therefore, for example, if the user wants to change the angle or the scale factor to see the region of interest such as the diseased area of the observation target, the user must wait for a long time until a large calculation amount image for the whole display screen including the region of interest is generated and is displayed each time the user operates the apparatus.